With the tapering process described here Dave Moulton's Blog - Dave Moulton's Bike Blog - Forming tapered tubes for bicycle frames a 19mm straight tube should weigh the same as a 19/15 tapered tube of the same length because the tapering described in the link above increases the wall thickness at the tapered end. Is that true and are tapered tubes actually made with the process shown in the video in the link above?
Another method used is called “swaging” (it’s actually mentioned in the comments section in the Dave Moulton post.
HERE’s a video showing that process. Not sure exactly what goes on inside that machine.
And HERE is a link to a company that makes such machinery for the bicycle industry.
As for the weight, if no material is removed or added, then the weight must stay the same.
It’s certainly true that the wall is thicker at the small OD end of a tapered tube. At least that’s my experience from when I’ve cut and mitered chainstays for the frames I’ve built.
2 Likes
This is certainly true (weight stays the same) for the method from the video at the top of the Dave Moulton blog (https://m.youtube.com/watch?v=ZAnzRRjKPpc&embeds_referring_euri=http%3A%2F%2Fdavesbikeblog.squarespace.com%2F&source_ve_path=MjM4NTE) because the tube is fixed in place.
But the tube seems to not be fixed in place here https://m.youtube.com/watch?v=Eq2g8VVcncU so for example a 19/15 tapered tube could be lighter than a 19mm straight tube.
That would mean your tubes were either made with the method from the video at the top of the Dave Moulton or that if they were “swagged” that the tube keeps it’s length also with this tapering method.
My preliminary conclusion is that a tapered tube has the same weight as the non tapered tube from which the tapered tube is made if both have the same length.
1 Like
As I said, if no material is removed or added in the process, the weight of the tube will be the same.
The overall length of the tube before and after shaping has no relevance whatsoever.
In other words, if the total volume of the metal that makes up the tube does not change, basic physics tells us the weight will stay the same.
One litre of water will weigh the same regardless of the shape of the vessel.
A paper plane folded from a single sheet of paper will weigh the same as the original sheet of paper.
1 Like
True. Note though that I was comparing tubes of the same length:)
Let’s analyse this:
My preliminary conclusion is that a tapered tube has the same weight as the non tapered tube from which the tapered tube is made.
Yes. So far you are correct.
if both have the same length.
This part is irrelevant.
1 Like
Let’s take a step back and look at the bike as a system.
The whole purpose of tapering a chainstay is so that it can land on a smaller dropout. The dropout size will have a larger weight difference than the tapered chainstay.
2 Likes
It’s irrelevant if the tapering method used doesn’t lengthen the tube. But if for example the tapering method from the video you posted makes the tapered tube longer than the non tapered tube it is coming from and you would then cut the tapered tube back to the length of the non tapered tube then the tapered tube would be lighter than the non tapered tube and then the length would matter.
BUT the only way to come to the conclusion that I came to (the first have of it) is if the tapering method does not increase the length of the tube and then the length indeed is irrelevant. So you are right 
This is the first time you mention cutting the tube.
Cutting the tube = removing material = said tube will be made lighter.
1 Like
In my initial post I said “of the same length”. So if tapering leads to tube lengthening (and the original question was implicitly about whether that is the case or not) and we want to compare tubes of the same length then the tapered tube needs to be cut. So tube cutting was implicit…
This is mostly a logic game we are playing here It would be much more interesting to know what tapering method is actually used for 80+% of tapered tubes and whether the tapering is done by thickening the tapered end or by lengthening the tube.